The present invention relates to measurement systems and, more particularly, to optical measurement or inspection systems for measuring contemporaneously several dimensions of or for measuring large objects.
The advantages of automatic determination of one or more dimensions of parts at an inspection position are well known, and to achieve those advantages various systems have been designed to inspect parts whose general shape and size are predetermined, the parts typically being mass-produced items. Noncontact systems based on optical scanning, for example, have been used to determine certain dimensions and are particularly useful for measuring small parts automatically. In one typical optical systems, a production item or workpiece is positioned within the field of view of an optical scanner which cooperates with a programmed processor to take sequentially any of a variety of measurements on the part. One limitation of such a system is the inspection and accurate measurement of large objects. The drift of the optical scanner over a small range is within acceptable limits, but when viewing a large object, the effect of drift reaches significant proportions. For example, if an optical scanner with a 1% drift were scanning an object having a 3 inch diameter, the 30 mils uncertainty of the measured dimension is unacceptably large.
A further problem faced by optical scanning measurement systems is that a large number of measurements must be taken without slowing or interrupting the production rate. One approach of the prior art to the optical measurement of large workpieces uses solid state linear arrays. The workpiece is positioned between a light source and a set of linear arrays, illustratively 2000 in number, each of which is a semiconductor chip having a window measuring approximately one-half mil in width. Each such linear array is coupled to a processor by a signal conditioner. The cost of a first channel is typically from about $3,000 to about $9,000, and although with multiple channels the cost per channel can drop to $1,000 per channel. With ten or more channels, however, the cost is still very high.
In another prior art approach, a laser generated narrow beam is deflected by an object, typically hexagonal in cross section, rotating at a known speed and having mirrored surfaces so that the beam sweeps a large line. The workpiece is positioned between the rotating object and a central optical receptor so that the beam illuminates the central receptor until the workpiece intercepts the beam, whereby the dimensions of the workpiece are ascertained from the "dark" time. The central receptor can be a photodiode or a solar cell, typically. One problem ailing such a system is its lack of versatility.
It is therefore an object of the present invention to overcome the scanner drift problem in an optical electronic measurement system which can measure large workpieces accurately.
It is a further object of the present invention to provide a versatile noncontact measurement system which can make several measurements quickly and at a reasonable cost.